1. Field of the Invention
The present invention is directed to scanning probe microscopes (SPMs), including atomic force microscopes (AFMs), and more particularly, to tuning the AFM for optimum operation.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employs a probe device 12 including a probe 17 having a cantilever 15. A scanner 24 generates relative motion between the probe 17 and a sample 22 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
Notably, scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in application Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
In a common configuration, probe 17 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 17 to oscillate at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever 15. Probe 17 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 17 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 17 but may be formed integrally with the cantilever 15 of probe 17 as part of a self-actuated cantilever/probe.
Often, a selected probe 17 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 17, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 17, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are processed by a signal processing block 28 (e.g., to determine the RMS deflection of probe 17). The interaction signal (e.g., deflection) is then transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 17. In general, controller 20 determines an error at Block 30, then generates control signals (e.g., using a PI gain control Block 32) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 17. The control signals are typically amplified by a high voltage amplifier 34 prior to, for example, driving scanner 24. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, As, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used. Controller 20 is also referred to generally as feedback where the control effort is to maintain a constant target value defined by the setpoint.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller 20 and manipulates the data obtained during scanning to perform data manipulation operating such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving the sample and/or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one practical mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe, or harmonic thereof. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e., the force resulting from tip/sample interaction, typically by controlling tip-sample separation. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
TappingMode AFM is a lower force technique and is the most widely used mode of AFM operation to map sample surfaces, especially for delicate samples. The typical force of the tip on the sample is about a few nN to tens of nN. Again, by oscillating the tip, rather than dragging the tip, the shear forces are minimized. That said, TappingMode AFM suffers from a drawback in that it is difficult to control the normal force acting on the sample surface. The user typically tries to select a setpoint that is only a small variation from the free air deflection/amplitude of the probe in order to minimize tip-sample interaction forces to get the best reproduction of the sample profile. The dilemma, especially for soft samples, is that if the imaging force is too low, the tip will not track the sample properly (i.e., maintain interaction with the sample during the scan), while if too high, damage/deformation of the sample may lead to an image that does not accurately reflect surface topography. Overall, the better this force can be controlled (i.e., the lower it can be maintained) the less chance of sample and/or tip damage, and thus resolution can be improved. And in this regard, an appropriate operating frequency, as well as amplitude and phase, needs to be selected for optimum operation.
More specifically, in order to operate in TappingMode (and variants thereof, such as PFT Mode), the AFM needs to provide efficient probe control. To do so, the frequency of oscillation, as well as the amplitude must be properly selected. Ideally, the probe is driven to oscillate at or about its fundamental resonant frequency for optimum AFM operation. Because the fundamental resonant frequency is unknown for each probe (even among probes fabricated from the same substrate (wafer)) and application, the operator of the AFM in TappingMode needs to initially determine this value. Classically, the amplitude response of the probe is plotted as a function of frequency as the frequencies are swept over a range. The resulting plot (sometimes referred to herein as an amplitude response curve) would typically include one or more amplitude peaks. It has been generally understood that one of these peaks, the largest typically, corresponds to the fundamental resonant frequency of the probe. The user would also adjust the drive amplitude to achieve the desired oscillation amplitude for the user's application. The user would do the same for the phase of the drive.
With respect to the selection of the peak, this method of determining the operating frequency is well accepted in the AFM community, and in many cases, it produces an accurate result. For instance, for operation in air, in which the Q of the cantilever is large, the amplitude response curve typically includes a sharp peak that is indicative of the fundamental resonant frequency of the probe. For operation in fluid, however, in which the Q is much lower due to fluid damping, the peak associated with the probe resonant frequency is wide and thus can be difficult to identify.
This tuning process has been automated in modern instruments, with start and end frequencies selected by the user, and the system identifying the highest amplitude peak associated with probe resonance. As noted above, known “auto-tune” algorithms have proven to be robust for TappingMode in air. This is primarily due to the fact that the quality factor (Q) of the probe is highest in air and therefore the assumption regarding the largest peak 52 of the frequency swept amplitude response 50 corresponds to the fundamental resonant frequency of the probe. (See, e.g., FIG. 2) The vertical line 54 represents the actual operating frequency of the probe due to a preset offset selected by the user. The offset is often positioned to the left of the largest peak of the tuning curve (i.e., amplitude response), at a lower frequency. This offset works to counter the damping affects experienced by the probe when the probe approaches the sample surface, thus allowing the system to maintain stable operation.
Though capable of providing stable operation, known automatic tuning methods also have drawbacks. For instance, the user defined parameters for conducting the tune may create problems. In one case, the user sets Start and/or End frequencies for the sweep. If, for example, the probe has a fundamental resonant frequency of 60 kHz, and the user sets the End frequency at 500 kHz, the automated tuning function may set the drive frequency to around 400 kHz, which exhibits an amplitude peak at an overtone frequency, rather than at the fundamental resonance.
Moreover, the setup of the AFM instrument may create problems identifying the resonance of the probe. For instance, the mechanical coupling between the probe holder and the probe substrate is often not ideal (even a small particle between the substrate and probe holder can compromise the system's ability to identify the probe resonance). If so, the amplitude response may include multiple peaks that make it difficult for the auto-tune algorithm to distinguish the probe resonant frequency. These peaks may also be caused by vibration of parts of the AFM other than the cantilever of the probe. In the end, the highest peak selected by the automatic tuning method may not correspond to the probe oscillation at all. See, for example, FIG. 3 in which a tuning curve 60 shows the fundamental resonant frequency of the probe at a peak 62 corresponding to a frequency of about 1.67 MHz, even though a larger peak 64 of the tuning curve corresponds to a frequency of about 1.85 MHz. Operating at this latter frequency, which would be selected by a typical auto-tune algorithm, would most likely prevent stable operation of the AFM. For a discussion regarding miscellaneous resonances present in an AFM environment, please see, Rabe et al., Influence of the cantilever holder on the vibrations of AFM cantilevers, Nanotechnology 18 (2007).
Moreover, when operating TappingMode in fluid, multiple peaks (and peaks which are more rounded and therefore less easily correlated to the fundamental resonant frequency of the probe) are often present in the swept amplitude response (tuning curve) due to fluid damping of the cantilever (i.e., cantilever Q is smaller). Again, the highest peak in the tuning curve may not necessarily correspond to the resonant frequency of the probe. (See, e.g., FIG. 4 in which the largest peak 72 of the tuning crave 70 at about 200 kHz does not correspond to the fundamental resonant frequency of the probe.)
In all these cases, the automatic tuning method is at risk of failing. And if the probe is driven at a frequency other than the resonant frequency, poor probe response can lead to the acquisition of poor SPM images. In addition, when operating significantly off resonance, probe tips and/or samples can be damaged due to poor force control. Unfortunately, it is not always obvious to the user that the tune was not performed correctly. Up until now, it has been understood that identifying the largest peak of the frequency swept amplitude response of the probe provided the best indication of the fundamental resonant frequency of the probe and thus the proper operating frequency.
Another method of identifying one of characteristics of the AFM probe involves measuring the thermally induced response of the probe. This technique is most often used to determine the spring constant of the probe, and includes plotting the displacement power spectrum as a function of frequency. Unlike a typical auto-tune apparatus and method such as that described above, in which a piezo-stack actuator is employed to drive the probe cantilever into oscillation in response to a drive signal, the only drive force to the cantilever is passive thermal energy, when using the thermal technique. As the probe responds to the thermal energy input, the probe deflects and the system captures the deflection signal with sampling rate of up to 50 MHz and does so for a selected period of time. After collecting the thermal response data in this way, a software module may be provided to fit the peak in the resulting curve with, for example, a simple harmonic model. The result (80 in FIG. 5) may be integrated to obtain the mean square amplitude. From this, the user can calculate the cantilever spring constant value while also gleaning resonant frequency and Q value information associated with the cantilever. See, for instance, B. Ohler, Cantilever spring constant calibration using laser Doppler vibrometry, Review of Scientific Instruments 78, 063701 2007.
Notably, generating a thermally induced displacement power spectrum curve related to an AFM probe is a computationally intense endeavor. Nonetheless, such thermal plots may be used in conjunction with the conventional tuning curves (described above) as a “check” to qualify certain AFM functions, such as automatic probe exchange apparatus and methods. In a typical AFM instrument, however, the thermal power and the operating frequency tuning function are separate and not used at the same time.
Because of the problems associated with imperfect AFM setup (e.g., mechanical resonances other than that of the oscillating probe being introduced) and challenging imaging environments (e.g., fluid) a more reliable solution for identifying an operating frequency that yields robust AFM operation was needed.